Discussion on Charging Control Strategy for Power Battery at Low Temperatures

In the case of electric vehicles, due to the charging current limitation of lithium battery at low temperatures (below -20°C), it has been proposed to heat the battery pack up to a suitable temperature range before charging through a liquid-heating plate with PTC. However, at a low state of charge (SOC), there is a question which one could take the place of battery pack to supply power for PTC when heating. So that off-board charger (OFC) has been considered to supply power for PTC in this article. In order to control the current charging into the battery pack as less as possible at low temperatures, three control schemes of battery management system (BMS) are proposed and compared. Scheme 1: BMS controls the value of charging current request close to the working current of PTC. Scheme 2: BMS controls the value of charging voltage request to reach a state of relative balance. Scheme 3: BMS disconnects the pack from the charger and keeps the connection between PTC and charger. The functions of the three schemes above have been verified by simulation tests and vehicle tests. The results showed: the first two schemes can supply the power for PTC successfully. However, the third one can’t keep PTC working properly due to a power-down fault judged by OFC. As a result, by comparison, Scheme 2 (the voltage control scheme) can more effectively control the current charging into the battery pack as less as possible.